Polypropylene is a synthetic resin having high rigidity and mechanical strength. Since uniaxially or biaxially stretched polypropylene has greatly improved mechanical strength, transparency and cold resistance, it is widely used in the production of a stretched tape, yarn or biaxially stretched film.
A catalyst system comprising titanium trichloride prepared by reducing titanium tetrachloride with an organoaluminum compound and activating the reduction product by a specific method, and an organoaluminum halide is used with advantage in commercial production of polypropylene because propylene polymerizes at a fast rate and only a small amount of an amorphous polymer having low commercial value is produced as a by-product. But recent studies have shown that polypropylene produced using such catalyst is low in stretchability and is apt to develop cracking upon stretching at high ratio, which results in insufficient improvement in the physical properties of the polymer by stretching.
It is generally known to copolymerize propylene and ethylene using a catalyst system comprising titanium trichloride and an organoaluminum halide. One reason to copolymerize propylene and ethylene is to reduce the melting point of polypropylene as much as possible so that a film of the propylene copolymer has good heat sealability and other properties. Therefore, to reduce the melting point of the propylene copolymer, it is necessary that the copolymer contain at least 1.5 wt% of ethylene. However, the propylene copolymer having incorporated therein at least 1.5 wt% of ethylene for improving its heat sealability provides only a film having very poor mechanical properties, particularly, very low Young's modulus. Another defect with such propylene copolymer is that its stretchability has a tendency to decrease with the increase in the ethylene content.